The present invention relates to a "passive interrogator label system" (PILS); that is a system comprising an interrogator for transmitting an interrogation signal, one or more "labels" or passive transponders which produce a reply signal containing coded information in response to the interrogation signal, and a receiver and decoding system for receiving the reply signal and decoding the information contained therein.
A passive interrogator label system of the type to which the present invention relates is disclosed in the U.S. Pat. No. 3,273,146 to Horwitz, Jr.; U.S. Pat. No. 3,706,094 to Cole and Vaughan; U.S. Pat. No. 3,755,803 to Cole and Vaughan; and U.S. Pat. No 4,058,217 to Vaughan and Cole. In its simplest form, the systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. Further "tap" transducers arranged at prescribed, spaced intervals along this path convert the acoustic wave back into electric energy for reconversion into electrical energy by the launch transducer. The presence or absence of tap transducers at the prescribed locations along the acoustic wave path determines whether a reply pulse will be transmitted with a particular time delay, in response to an interrogation pulse. This determines the informational code contained in the transponder reply.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, preferably at the same location as the interrogating transmitter, and the information contained in this response to an interrogation is decoded.
In the patent application Ser. No. 509,523, filed June 30, 1983, for "System for Interrogating a Passive Transponder Carrying Amplitude and/or Phase Encoded Information" of P. Nysen, H. Skeie and D. Armstrong, it is proposed to provide an interrogator for transmitting a first, interrogation signal having a first frequency which successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905 to 925 MHz, a frequency band which is freely available in many parts of the world for short range transmission.
The remote, passive transponder associated with this interrogator receives the first signal as an input, and produces a second, reply signal as an output. Signal transforming means within the transponder convert the first signal in such a way as to impart a known informational code in the second signal which is associated with and identifies the particular passive transponder.
Associated with the interrogator of the system is a receiver for receiving the second signal from the passive transponder and a mixer, arranged to receive both the first signal and the second signal, or signals derived therefrom, for mixing together these two signals and thereby producing a further signal. This further signal may, for example, contain the sum and the difference frequencies of the first and the second signals, respectively.
Finally, the proposed system includes a signal processor responsive to the signal produced by the mixer for detecting the frequencies contained in this signal to thereby determine the informational code associated with the passive transponder.
In systems of the this general type, all of the tap transducers are arranged in series along a single path of travel for the surface acoustic wave. With this arrangement, however, surface acoustic wave reflections from the various tap transducers are picked up by other tap transducers and converted into spurious electrical signals. When the number of tap transducers is large, for example, up to 16 in a typical passive transponder, these spurious signals add up to a severe degradation in the desired informational component of the reply signal.